The influence of inorganic matrices on the decomposition of Eucalyptus litter

Abstract

The decomposition of Eucalyptus litter (EL) in the presence and absence of inorganic matrices [sand (S), sand+kaolin (S+K), loamy sand (LS)] with and without added N (urea) was followed over 48 weeks using chemical and spectroscopic means. At the end of the incubation, the residual organic matter in different density and particle size fractions was examined. Urea addition inhibited the mineralisation of C from the litter in all treatments except EL+S+N, whereas the inorganic matrices had little influence on mineralisation. Solid state ¹³C CP/MAS NMR spectra of the whole samples suggested there were no differences in the degree of decomposition (as determined by the O-alkyl-C : alkyl-C ratio) between the different treatments, despite significant differences in the amount of C mineralised.

Inorganic matrices and N addition had a significant influence on the yields of the 2 `light" fractions [D < 1·6 and 1·6 < D < 2·0, where D (Mg/m³) is density]. There were also differences in the degree of decomposition (as determined by the O-alkyl-C : alkyl-C ratio) of the D < 1·6 fractions from the different inorganic matrix treatments. However, there were no differences in the degree of decomposition of the 1·6 < D < 2·0 fractions. The C: N ratios and O-alkyl- C: alkyl-C ratios of the different fractions revealed that the order of fractions from the least decomposed to the most decomposed was (D < 1·6) < (1·6 < D < 2·0) < clay+silt < clay.

The NMR spectra of the whole samples suggest that a reaction between aromatic-C and urea occurred during the first week of the incubation which may have rendered the N unavailable to microorganisms. This reaction requires further investigation.

The results were quite different from a similar study on the decomposition of straw. These differences suggest that, for high quality substrates, physical protection by inorganic matrices is the limiting factor to decomposition, whereas for low quality substrates, chemical protection is the limiting factor.